The present invention relates to silicon-on-insulator (SOI) substrates for use in the semiconductor industry for fabricating integrated circuits (ICs), and more particularly to a method for improving the thickness uniformity of the top superficial Si-containing layer of a SOI substrate. The SOI substrate may be a preformed SOI substrate or a SOI substrate that is prepared in-situ.
Silicon-on-insulator (SOI) thickness and thickness uniformity requirements for future generation of complementary metal oxide semiconductor (CMOS) devices and circuits are becoming highly demanding. Typically, SOI thickness targets are from about 100 to about 2500 xc3x85 with a thickness uniformity of about xc2x12% over a 200 mm Si wafer. This is because the threshold voltage, Vt, of today""s generation of CMOS devices can be strongly dependent on the SOI thickness, especially in the low thickness range.
Thickness uniformity of present commercially available SOIs made either by conventional SIMOX or bonding is quite inadequate to meet the required thickness control targets. A typically SOI substrate of the prior art is shown, for example, in FIG. 1. Specifically, the SOI substrate shown in FIG. 1 comprises bottom Si-containing substrate 10, buried oxide (BOX) region 12 formed on the bottom Si-containing substrate, and superficial Si-containing layer 14 having a surface with roughness variation. Note that the area above the dotted line represents the thickness non-uniformity in the superficial Si-containing layer that is formed from conventional SIMOX and bonding processes. Due to this non-uniformity, the thickness in layer 14 may vary randomly across the SOI substrate.
Ideally, it would be beneficial if the superficial Si-containing layer has thickness uniformity across the entire length of the SOI substrate. In the prior art, it is known to control thickness uniformity of a SOI substrate by thinning in hydrogen, chemical mechanical polishing (CMP), or low temperature oxidation (at temperatures below about 1100xc2x0 C.). The aforementioned prior art approaches to achieve thickness uniformity do not demonstrate sufficient thickness control needed with today""s generation of CMOS devices.
Thus, there is a need for developing a method in which thickness uniformity can be readily achieved utilizing processing steps that do not adversely affect the SOI substrate. Moreover, a method is required which demonstrates sufficient thickness control to permit the use of SOI substrates with today""s and future generation of CMOS devices.
One object of the present invention is to provide a method of improving the thickness uniformity of a SOI (preformed or in-situ) substrate.
Another object of the present invention is to provide a method of forming a SOI substrate having a SOI thickness of less than about 1000 xc3x85.
Another object of the present invention is to provide a method of improving the thickness uniformity of SIMOX or bonded SOI substrates.
A yet further object of the present invention is to provide a method of improving the thickness uniformity of a SOI substrate that employs processing steps that do not adversely affect the SOI substrate.
A still further object of the present invention is to provide a method for achieving thickness uniformity of a SOI substrate which demonstrates sufficient thickness control to permit the SOI substrates to be used in fabricating high-performance semiconductor devices and circuits.
An even further object of the present invention is to provide a method of improving the thickness uniformity in a self-adjusting manner by using an internal thermal oxidation (ITOX) mechanism.
These and other objects and advantages are achieved in the present invention by subjecting a SOI substrate having thickness non-uniformity to high-temperature oxidation step. During the high-temperature oxidation the following two phenomena are believed to be taking place which aid in improving the thickness uniformity of the SOI substrate: (i) surface oxidation of the superficial Si-containing layer (i.e., the Si-containing layer present atop the buried oxide (BOX) region); and (ii) internal thermal oxidation (ITOX), i.e., diffusion of oxygen via the superficial Si-containing layer into the interface that exists between the BOX and the superficial Si-containing layer.
Regions of the SOI substrate which have thicker Si get less ITOX, but more surface oxidation creating a thicker surface oxide (and hence thinner superficial Si-containing layer). However, regions of the SOI substrate which are thinner get more ITOX and have thinner surface oxide (and hence a thicker superficial Si-containing layer). This mechanism continues to self-adjust the SOI substrate thickness with oxidation time to improve the SOI thickness uniformity. The same mechanism improves the surface roughness. It has been observed that the closer the oxidizing surface is to the BOX, the more uniform the superficial Si-containing layer 14 becomes.
A first method of the invention which is employed with a preformed SOI substrate (SIMOX or bonded) comprises the steps of: subjecting a preformed SOI substrate having thickness non-uniformity to a high-temperature oxidation process that is capable of improving the thickness uniformity of said SOI substrate, wherein said high-temperature oxidation is carried out at a temperature of about 1000xc2x0 C. or above, preferably 1250xc2x0 C. or above and in an oxygen-containing ambient that promotes surface oxide growth and internal thermal oxidation.
In accordance with the present invention, the preformed SOI substrates employed in the present invention include SOI substrates that are formed by conventional SIMOX or bonding techniques. Included within this definition are unpatterned or patterned SOI substrates that have a single or multiple buried oxide regions formed therein.
A second method of the present invention is directed to an in-situ process of preparing a SOI substrate wherein the inventive high-temperature oxidation step is used as a soak cycle prior to annealing. Specifically, the second method of the present invention comprises the steps of:
(a) implanting oxygen ions into a surface of a Si-containing substrate, said implanted oxygen ions having a sufficient concentration to form a buried oxide region in said Si-containing substrate;
(b) soaking said Si-containing substrate containing implanted oxygen ions in an oxygen-containing ambient at a temperature of about 1000xc2x0 C. or above, preferably 1100xc2x0 C. or greater for a time period of about 0.5 hours or greater; and
(c) annealing said soaked Si-containing substrate containing said implanted oxygen ions under conditions that are capable of forming a SOI substrate having said buried oxide region which electrically isolates a superficial Si-containing layer from a bottom Si-containing layer, said SOI substrate having thickness uniformity.
In a third method of the present invention, the high-temperature oxidation step is used in a conventional SIMOX process following the annealing step. When this aspect is employed, a SIMOX process is provided that comprises the steps of:
(a) implanting oxygen ions into a surface of a Si-containing substrate, said implanted oxygen ions having a sufficient concentration to form a buried oxide region in said Si-containing substrate;
(b) annealing said Si-containing substrate containing said implanted oxygen ions under conditions that are capable of forming a SOI substrate having said buried oxide region which electrically isolates a superficial Si-containing layer from a bottom Si-containing layer, said SOI substrate having thickness non-uniformity; and
(c) subjecting said SOI substrate having thickness non-uniformity to a high-temperature oxidation process that is capable of improving the thickness uniformity of said SOI substrate, wherein said high-temperature oxidation is carried out at a temperature of about 1000xc2x0 C. or above, preferably 1250xc2x0 C. or above, and in an oxygen-containing ambient that promotes surface oxide growth and internal thermal oxidation.
Another aspect of the present invention relates to a SOI substrate (or material) having thickness uniformity across the entire surface of the substrate, which is provided by one of the aforementioned methods. Specifically, the SOI substrate of the present invention comprises:
a buried oxide region that electrically isolates a superficial Si-containing layer from a bottom Si-containing layer, wherein said superficial Si-containing layer has a thickness which is substantially uniform across the entire substrate.
Because each of the methods of the present invention disclosed herein goes through an internal thermal oxidation mechanism, the buried oxide (BOX) regions of the present invention exhibit high structural as well as electrical qualities as compared to buried oxide regions formed using conventional ion implantation, wherein the inventive high-temperature heat treatment method of the present invention is not employed.
The term xe2x80x9chigh structural qualityxe2x80x9d is used herein to denote a SOI substrate which has little or no etch pitch density (less than about 1E5 cmxe2x88x922); little or no top Si roughness (on the order of less than about 6 xc3x85 root mean square), Si-buried oxide interface undulations of less than about 200 xc3x85 as measured by TEM (transmission electron microscopy) or SEM; and a low HF-defect density of less than about 1 cmxe2x88x922.
The term xe2x80x9chigh electrical qualityxe2x80x9d is used herein to denote a structure wherein the buried oxide breakdown field is high (greater than about 6 megavolts per cm); the buried oxide minibreakdown voltage is high (greater than about 5 megavolts per cm); and the buried oxide leakage at a given voltage is low (less than about 1 nanoAmp per cmxe2x88x922).